...initiates electrical signals that
activate the hydraulic systems and cause the control surfaces to
be moved in a prescribed manner. The fly-by-wire system is
lighter, simpler, and more precise than the older mechanical
systems, but it does raise questions relating to electrical system
reliability. In the F-16, redundancy is provided in the electrical
generating and distribution equipment, and four dedicated
sealed-cell batteries give transient electrical power protection
for the fly-by-wire system. Two completely separate and
independent hydraulic systems supply power for actuation of the
aerodynamic control surfaces and other utility functions.
Another novel feature in the control
system of the F-16 is the incorporation of "relaxed static
stability." This means that the inherent longitudinal stability is
reduced, to a level traditionally thought to be unacceptable, by
moving the aircraft center of gravity to a point very near the
aerodynamic center of the aircraft. Tall load and associated trim
drag are reduced by this process. Compensation for the loss in
inherent aerodynamic stability is provided by a combination
electronic-hydraulic stability augmentation system that senses
uncalled-for [342] departures from the intended flight condition and
injects corrective signals into the flight control system.
Finally, the arrangement of the pilot's
control stick is a radical departure from standards that trace
their origin to the early days of World War I. Traditionally, the
fighter pilot's control stick used for actuation of the ailerons
and elevators has consisted of a lever mounted on the floor of the
cockpit between the pilot's legs. (There have, of course been many
variations in the detail design of the control stick.) On the
F-16, the traditional control stick has been replaced by a short
"side-arm controller" mounted on the right-hand console of the
cockpit. The side-arm controller is a small-displacement
pressure-sensitive handle that, together with the fly-by-wire
system, gives the pilot the ability to exercise very precise
control of the aircraft. To help prevent unwanted commands to the
control handle the pilot rests his right arm in a carefully
designed support. In order to increase the pilot's tolerance to
forces his seat is inclined 30 in the rearward direction
with
The data in table V show the design gross weight of the F-16A to be 23
357 pounds, or only about half that of the F-15C. However, wing
loading and thrust- to-weigh t ratio of the two aircraft are
nearly the same. Little performance information is available for
the F-16A; the limited data in table V do show, however, a maximum Mach number of 2.02 at
40 000 feet and a ferry range of 2535 miles.
Originally conceived as a simple
air-superiority day fighter, the aircraft was armed for that
mission with a single six-barrel Vulcan 20-mm cannon and two
Sidewinder missiles, one mounted at each wingtip. Over the years,
however, the mission capability of the aircraft has been extended
to include ground-attack and all-weather operations With full
internal fuel, the aircraft can carry up to 12 000 pounds of
external stores including various types of ordnance as well as
fuel tanks.
The F-16 Fighting Falcon is an advanced
and innovative fighter that, like the F-14 and the F-15, will be a
part of the fighter scene for many years.
British Aerospace AV-8A
Harrier
Discussed next is a totally unique
aircraft that has an operational versatility unmatched by any
other fighter in the western world. The British Aerospace Harrier
can take off and land vertically like a helicopter but, unlike the
well-known rotary-wing machine, accomplishes this [343] vertical-flight
operation by means of a specially designed jet engine that is also
able to propel the aircraft in forward flight at Mach numbers as
high as 0.95 at an altitude of 1000 feet. An early prototype,
known as the Hawker P-1127, flew in 1960 and was the basis of a
more refined aircraft that appeared later. Known as the Kestrel, a
number of these aircraft were employed during the mid-1960's in a
joint military evaluation of the VTOL fighter concept conducted by
the governments of the United States, the United Kingdom, and the
German Federal Republic. In the 1970's, the aircraft now called
the Harrier entered the active inventory of several air forces. Of
the same basic design, the progression from P-1127 to Kestrel to
Harrier was characterized by increased power, weight, and
performance.
The Kestrel is shown in figures 11.39 and
11.40. This particular aircraft served in the joint United States,
British, and German evaluation; it was later used in extensive
flight studies at NASA's Langley Research Center. Today it may be
seen in the National Air and Space Museum in Washington, D.C. A
Harrier in service with the U.S. Marine Corps is shown in figure
11.41. The designation AV-8A is used to describe these
aircraft.
The Rolls-Royce (Bristol division) Pegasus
turbofan engine is the key to the great versatility of the
Harrier. Unlike other jet engines with only one jet-exhaust
nozzle, the Pegasus has four exhaust nozzles; two....
Figure 11. 39 - British
Aerospace Kestrel single-engine VTOL jet fighter. [NASA] [Original photo was in color, Chris Gamble,
html editor]
[344] Figure 11.40 - British Aerospace Kestrel in hovering
flight. [NASA] [Original photo was
in color, Chris Gamble, html editor]
Figure 11. 41 - British
Aerospace Harrier single-engine VTOL jet fighter. [ukn] [Original photo was in color, Chris Gamble,
html editor]
...are located on each side of the engine.
The two front nozzles discharge unheated air compressed by the
fan, and the rear nozzles discharge the hot jet exhaust. A
rotating cascade of vanes is used in each nozzle to vector the
thrust from a horizontal direction for high-speed flight to a
vertical direction for hovering and vertical takeoff and landing.
Intermediate positions are used for short takeoff and landing
(STOL) and [345] for maneuvering in combat situations. (This latter
technique is referred to as VIFF, vectoring in forward flight.)
The use of VIFF to enhance aircraft maneuverability and hence
combat effectiveness was pioneered in flight studies at the
Langley Research Center in the late 1960's and early 1970's. For
rapid deceleration, the nozzles can actually be rotated past the
vertical position to about 98°. The thrust-vectoring nozzles
can be seen in the side of the fuselage in figure 11.39.
Another key element in the Harrier concept
is the method for controlling the aircraft. When operated as a
conventional airplane, the usual ailerons, rudder, and horizontal
tail are used to generate aerodynamic control moments about the
roll, yaw, and pitch axes, respectively. In hovering flight and at
low forward speeds, however, the aerodynamic controls are
ineffective, and reaction jets are used to provide the necessary
control moments. At intermediate speeds, both reaction jets and
aerodynamic controls are used. As indicated in figure 11.39, pitch
jets are located at the nose and tail of the fuselage, a roll jet
is at each wingtip, and a yaw jet is located behind the tail. The
reaction jets utilize compressed air from the high-pressure engine
compressor and respond in a proportional fashion to conventional
movements of the control stick and rudder pedals. The control jets
come into operation automatically when the thrust-vectoring
nozzles are rotated to any angle in excess of 20°. Control of
the thrust-vectoring nozzles is exercised by a lever in the
cockpit located alongside the throttle.
Although the engine and reaction control
system are the key elements that give unique operational
capability to the Harrier, the airframe itself exhibits several
interesting features. With 12° anhedral (negative dihedral),
the 34° sweptback wing is mounted on top of the fuselage;
like the wing, the all-moving horizontal tail has a large anhedral
angle (15°). The anhedral angles of the wing and horizontal
tail are intended to minimize the aircraft rolling moments due to
sideslip. Even so, at certain combinations of low speed and high
angle of attack, aerodynamic rolling moments greater than the
combined aerodynamic and reaction control power may occur if the
angle of sideslip is allowed to exceed a prescribed value. To
assist the pilot in maintaining the angle of sideslip within
acceptable limits, a small yaw vane that provides a visual
indication of sideslip angle is mounted on the fuselage just ahead
of the windshield.
The unusual landing gear of the Harrier is
designed to avoid interference with the engine and
thrust-vectoring nozzles. A single twowheel bogie is located in
the fuselage behind the engine, and a single steerable nose-wheel
is in front of the engine. Balancing outrigger [346] wheels mounted
at the wingtips retract into the reaction control fairings. (See
fig- 11.41.) The wing anhedral angle minimizes the length of the
outrigger landing-gear struts. Also evident in the figure are the
large side-mounted subsonic inlets that supply air to the 21
500-poundthrust engine.
The fighter version of the aircraft is
manned by a single pilot; a two-seat trainer with the full
military capability of the single seater is also available. As
with so many modern jet fighters, the Harrier is equipped with
zero-zero ejection seats; that is, crew escape is possible on the
runway at zero altitude and zero speed.
The data in table V for the AV-8A version of the Harrier show a design
gross weight of 18 000 pounds for VTOL operation and 26 000 pounds
for STOL use. For the design gross weight as a VTOL aircraft, the
thrust-to-weight ratio is 1.19 and the wing loading is 89.5 pounds
per square foot. Maximum speed is listed as Mach 0.95 at an
altitude of 1000 feet, and 2.38 minutes are required to reach 40
000 feet; service ceiling is 48 000 feet, and ferry range with
maximum external fuel is 2070 miles.
Primary mission of the Harrier as employed
by the Royal Air Force is that of a ground-attack fighter-bomber.
In this role, a variety of external ordnance with maximum weight
up to 5000 pounds may be carried, as well as two 30-mm cannons.
The Royal Navy employs the aircraft in a fleet air-defense role;
in this capacity, Sidewinder missiles are carried in addition to
the cannon and various external stores. In naval use, the Harrier
employs a short takeoff technique from a small carrier equipped
with a ski-jump launching ramp; after its mission and at a much
reduced weight, the aircraft makes a vertical landing on the
carrier. This mode of operation is referred to as STOVL, short
takeoff and vertical landing. Although generally available
information is far from complete, the Harrier was apparently
employed with great effectiveness in the Falkland Islands dispute
between Great Britain and Argentina in 1982.
At the present time, the British Aerospace
Harrier is used by the Royal Air Force and Royal Navy, the U.S.
Marine Corps, and the navies of Spain and India. By mid-1980,
about 304 aircraft had been produced or were on order; of this
number, 110 were in service with the U.S. Marine Corps (ref.
177). An improved version of the Harrier, known as the
AV-8B, is now being sought by the Marine Corps. If procured in
production quantity, this aircraft will be manufactured in the
United States by McDonnell Douglas under an agreement with the
British Aerospace Corporation.